1. Field of the Invention
The present invention relates to an optical filter, particularly to an optical filter which can be mounted in a planar lightwave circuit, etc.
2. Description of the Related Art
An optical filter can be manufactured by various methods depending on the objects and functions of the filter. Some functions include non-reflecting, high reflecting, bandwidth transmitting, or wave division multiplexing or demultiplexing.
Advancement in data communications and digital media has also led to the advancement in optical devices. An example can be seen in the development and use of planar lightwave circuits that has led to miniaturization and high integration of optical devices. The device performs many functions by integrating number of different types of optical filters.
FIG. 1 is a plain view of a planar lightwave circuit in which a conventional optical filter is mounted. Referring to FIG. 1, the planar lightwave circuit 100 includes first and second waveguides 111 and 112, a recess 120 positioned between the first and second waveguides 111 and 112, an optical filter 130 positioned in the recess 120. First and second waveguides 111 and 112, which provide paths for lights, are comprised of an upper clad, an active layer, and a lower clad stacked sequentially on top of a semiconductor substrate (not shown).
The recess 120 is formed by dicing. The first and second waveguides are separated by a predetermined distance due to the recess 120 that traverses through lower clad, the active layer, and the upper clad. The distance between the first and second waveguides is determined by the thickness of the optical filter 130 that is to be inserted.
A light proceeding through the first and second waveguides 111 and 112 is diversed in the recess at a predetermined angle. As the distance between the first and second waveguides 111 and 112 gets larger, more of the light gets diversed and more of the light gets lost. Particularly, if the separation caused by the recess in a conventional single mode waveguide exceeds 30 μm, the engagement loss is substantial as most of the light signal is diversed. Therefore, a general optical filter inserted in the recess 120 should have a thickness of about 15 to 30 μm, and an optical thin film filter which uses a polymide substrate which can have a thickness of under or over 10 μm.
The optical filter 130 can be formed on a substrate of a polymer material by depositing a plurality of dielectric mediums. The optical filter 130 is settled down in a recess 120 and adhered to the recess 120 by a thermosetting medium or ultraviolet rays of high polymer material capable of matching the index of refraction between the first and second waveguides 111 and 112. Epoxy-based or silicon-based material is used for the high polymer. The recess 120 is formed by sawing.
The optical thin film filter described heretofore, however, has several problems. First, it becomes more difficult to treat the film as the film's thickness decreases, and such difficulty leads to rise in manufacturing cost. Second, low thickness of the film exposes it to physical damages under a slight external force. Such exposure hinders the films from being positioned on the planar lightwave circuit to be processed in an optical axis arranging process.